1. Technical Field
The present invention relates to an idle tone dispersion device improving the resolution of a frequency digital signal conversion, and a frequency measurement device.
2. Related Art
As a method of converting an analog signal into a digital signal, a delta-sigma modulation method is known. In addition, in U.S. Pat. No. 6,362,769, a frequency measurement device is disclosed in which a frequency of a measured signal is measured by using the delta-sigma modulation method. Further, in U.S. Pat. No. 6,362,769, it is disclosed that a continuous output of a count value of a configuration without having a dead period corresponds to a first delta-sigma modulation signal corresponding to the level of the frequency of the measured signal in the frequency measurement device. In the following description, a converter for specifying the frequency of the measured signal by converting the measured signal by the delta-sigma modulation method using a reference signal is referred to as a Frequency Delta Sigma Modulator (FDSM).
In general, it is known that a cyclic quantization noise referred to as an idle tone occurs in an output of a delta-sigma modulator (for example, refer to Chapter 2.6 in Richartd Schreier, Gabor C. Temes.; Introduction to ΔΣ Type Analog/Digital Converter, pages 34 to 41, 2007). The idle tone is a noise directly connected to a degradation in measurement accuracy in frequency measurement using FDSM.
In Dag T Wisland, et al., ESSCIRC2002, pp. 687-690, 2002, a method is considered in which the measured signal is sequentially delayed and is supplied to a plurality of parallelized FDSMs, and the reference signal is supplied to the entire FDSM, and an output of a plurality of parallelized FDSMs is added, and thus the idle tone is suppressed.
However, when a frequency modulation signal is input into FDSM as the measured signal, a quantization noise component in the output of FDSM is overlapped with a baseband signal component of the measured signal. The frequency measurement device in Dag T Wisland, et al., ESSCIRC2002, pp. 687-690, 2002 is premised on the fact that even when the measured signal which is delayed is input into FDSM, the baseband signal component included in the output is not changed, but the quantization noise component included in the output is not correlated at the time of being compared between parallel outputs. Then, in order to minimize a circuit scale, when a configuration of FDSM is limited to be a bit stream output, and then the frequency of the measured signal is lower than a frequency of the reference signal, an effect of dispersing the quantization noise is maximized by designing each amount of delay to divide a half cycle of the measured signal.
However, in Dag T Wisland, et al., ESSCIRC2002, pp. 687-690, 2002, it is also reported that an expected improvement is not obtained in experiment verification.